HANOI, Vietnam (AP) — Vietnam has apparently dropped its objections to a dam that Laos is constructing on the Mekong River, saying Thursday that the neighboring country has made changes in the design to mitigate any negative downstream impact.

Laos officials reportedly said this week that construction of the Xaburi dam was going ahead, and they took several journalists and diplomats to the site of the proposed dam on Southeast Asia’s mightiest river. Construction of approach roads and support buildings has already begun.

Vietnam and Cambodia last year proposed a 10-year moratorium on any dams on the Mekong. They have expressed concern that the dam would kill fish and affect the livelihoods of millions of people living along their stretches of the river, which begins in China and empties into the South China Sea

Vietnamese foreign ministry spokesman Luong Thanh Nghi said Thursday that Laos had decided to build the $3.5 billion dam ‘‘after adjusting the project design to mitigate the impact on the downstream.’’

The U.S. on Monday criticized the decision to go ahead with the dam, citing feared negative environmental impact downstream.

Opponents say the dam in central Laos would open the door for a building spree of as many as 10 other dams on the 3,000-mile-long (4,800-kilometer-long) river.

Laos is one of Asia’s poorest nations and hydropower is already a key source of revenue. The project will generate electricity for sale to neighboring Thailand.